Fill level measurements of this type are applied in wide ranges of industrial measurements technology for fill level measurement of fill substances located in containers.
Two different measuring principles are known, which permit fill level to be measured by means of a measuring probe introduced into a container.
A first measuring principle is based on a travel time measurement. In such case, the fill level measuring device produces electromagnetic signals, which it transmits into the container along the measuring probe, in this case serving as a waveguide. A part of these electromagnetic signals is reflected from the surface of the fill substance, and its echo signal is received back after a travel time dependent on the fill level. The fill level measuring device determines the travel time which passes between the transmission of the signal and the receipt of the echo signal arising from the reflection off the surface of the fill substance. The determining of these travel times proceeds on the basis of known travel time measuring methods. In connection with guided electromagnetic signals, time-domain reflectometry (time domain reflection), for example, is used. In such case, a high-frequency pulse is transmitted, for example, according to the guided microwave method, along a Sommerfeld waveguide, a Goubau waveguide or a coaxial waveguide. If this electromagnetic signal strikes a surface of the fill substance in the container, at least a part of the signal is then reflected back due to the impedance jump existing at this media boundary. The received signal amplitude as a function of time represents the echo signal. Each value of this echo signal corresponds to the amplitude of an echo reflected at a particular distance from the transmitting and receiving element. The echo signals have clear maxima, which correspond to the portions of the electromagnetic signals in each case reflected off the surface of the fill substance. From the time difference between the transmission of the electromagnetic signal and the receipt of the maxima, the sought travel time is ascertained.
On the basis of the structural dimensions of the measuring arrangement—especially of the installed height of the fill level measuring device with respect to the container—and the propagation velocities of the electromagnetic signals in a medium located above of the upper fill substance, e.g. in air, there results from the travel time the fill height of the fill substance in the container, and therewith the fill level present in the container.
A second measuring principle is based on a capacitance measurement. In this case, the measuring probe serves as a capacitive probe, or as an electrode. It is inserted in the container, and the capacitance of the capacitor formed by the probe and the container wall surrounding it is measured. The measured capacitance corresponds to the sum of a basic capacitance of the empty container, the product of a fill-substance-specific capacitance increase factor of the fill substance, and the latter's fill height.
Classical fill level measuring devices for measuring fill level of a fill substance located in a container are known, in the case of which the travel time measurement principle is combined with the capacitive measuring principle in one measuring device. An example of this is the apparatus described in DE 100 37 715 A1 of the assignee for measuring fill level of a single fill substance located in a container. The apparatus includes a probe, which can be operated selectively as a capacitive probe of a classic capacitive fill level measuring device, as well as also as a waveguide of a classic fill level measuring device, working according to the travel time principle.
A further example is described in DE-A1 195 10 484. This application describes a fill level measuring device working according to the travel time principle. The device has a waveguide, and, in the waveguide, a metal inner conductor is provided, which serves as a capacitive probe.
Preferably, so-called coaxial probes are applied as measuring probes both for capacitive fill level measurement, as well as also for fill level measurement according to the travel time principle. These comprise an inner conductor and a shield conductor coaxially surrounding the inner conductor. Coaxial probes offer the advantage that the measurements performed therewith occur completely independently of the installed situation of the measuring probe in the container. Especially the shape and electrical properties of the container no longer have an influence on the measuring. At the same time, via the shield conductor, a maximum signal quality is achieved, influences from external disturbances and power losses are significantly reduced thereby.
In order to be able to apply such a coaxial probe for a capacitive fill level measurement and/or a fill level measurement according to the above described travel time principle, it is absolutely required that the inner conductor is galvanically isolated from the shield conductor. That is to say, even in the case of use in a container filled with an electrically conductive fill substance, no galvanic connection is permitted to occur between the inner conductor and the shield conductor. Such a galvanic connection would lead to a short circuit, which renders both capacitive fill level measurement as well as fill level measurement according to the travel time principle impossible.
At the same time, it must be assured that the shield conductor is electrically connected to a reference potential, preferably to ground. The reference potential forms for the capacitive fill level measurement a reference potential, with respect to which the capacitance measurement is performed. If the shield conductor were not at the reference potential, in the context of the capacitive fill level measurement, an isolated capacitance would be formed in the container by the inner conductor and the shield conductor. An isolated capacitance is not allowable, especially in applications in which special explosion protection measures must be made for safety reasons.
The shield conductor preferably lies at reference potential—preferably at ground—also in the case of fill level measurement according to the travel time principle. If this is not so, in the region of the coupling of the electromagnetic signal into the coaxial probe, impedance jumps can be present, which lead to strong input reflection of the electromagnetic signals in this region. These undesired input reflections lead to a markedly higher power loss.